A prodrug is a pharmacological substance (drug) that is administered in an inactive (or significantly less active) form. Once administered, the prodrug is metabolisedin vivo into an active metabolite. The rationale behind the use of a prodrug is generally for absorption, distribution, metabolism, and excretion (ADME) optimization. Prodrugs are usually designed to improve oral bioavailability, with poor absorption from the gastrointestinal tract usually being the limiting factor.

Additionally, the use of a prodrug strategy increases the selectivity of the drug for its intended target. An example of this can be seen in many chemotherapy treatments, in which the reduction of adverse effects is always of paramount importance. Drugs used to target hypoxiccancer cells, through the use of redox-activation, utilise the large quantities of reductaseenzyme present in the hypoxic cell to convert the drug into its cytotoxic form, essentially activating it. As the prodrug has low cytotoxicity prior to this activation, there is a markedly lower chance of it "attacking" healthy, non-cancerous cells which reduces the side-effects associated with these chemotherapeutic agents.

In rational drug design, the knowledge of chemical properties likely to improve absorption and the major metabolic pathways in the body allows the modification of the structure of new chemical entities for improved bioavailability. Sometimes the use of a prodrug is unintentional, however, especially in the case of serendipitous drug discoveries, and the drug is only identified as a prodrug after extensive drug metabolism studies.

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Prodrugs can be classified into two major types, based on their cellular sites of conversion into the final active drug form, with Type I being those that are converted intracellularly (e.g., anti-viral nucleoside analogs, lipid-lowering statins,), and Type II being those that are converted extracellularly, especially in digestive fluids or the systemic circulation (e.g., etoposide phosphate, valganciclovir, fosamprenavir, antibody-, gene- or virus-directed enzyme prodrugs [ADEP/GDEP/VDEP] for chemotherapy or immunotherapy). Both types can be further categorized into Subtypes, i.e. Type IA, IB and Type IIA, IIB, and IIC based on whether or not the intracellular converting location is also the site of therapeutic action, or the conversion occurs in the gastrointestinal (GI) fluids or systemic circulation (see Table 1).

Type IA prodrugs include many antimicrobial and chemotherapy agents (e.g., 5-flurouracil). Type IB agents rely on metabolic enzymes, especially in hepatic cells, to convert the prodrugs intracellularly to active drugs. Type II prodrugs are converted extracelluarly, either in the milieu of GI fluids (Type IIA), within the systemic circulation and/or other extracellular fluid compartments (Type IIB), or near therapeutic target tissues/cells (Type IIC), relying on common enzymes such as esterases and phosphatases or target directed enzymes. Importantly, prodrugs can belong to multiple subtypes (i.e., Mixed-Type). A Mixed-Type prodrug is one that is converted at multiple sites, either in parallel or sequential steps. For example, a prodrug, which is converted concurrently in both target cells and metabolic tissues, could be designated as a “Type IA/IB” prodrug (e.g., HMG Co-A reductase inhibitors and some chemotherapy agents; note the symbol “ / ” applied here). When a prodrug is converted sequentially, for example initially in GI fluids then systemically within the target cells, it is designated as a “Type IIA-IA” prodrug (e.g., tenofovir disoproxil fumarate; note the symbol “ - ” applied here). Many ADEPs, VDEPs, GDEPs and futuristic nanoparticle- or nanocarrier-linked drug moieties can understandably be Sequential Mixed-Type prodrugs. To differentiate these two Subtypes, the symbol dash “ - ” is used to designate and to indicate sequential steps of conversion, and is meant to distinguish from the symbol slash “ / ” used for the Parallel Mixed-Type prodrugs
[1],[2].

Carisoprodol is metabolized into meprobamate. Carisoprodol is not a controlled substance in the United States, but meprobamate is classified as a potentially addictive controlled substance that can produce dangerous and painful withdrawal symptoms upon discontinuation of the drug.